A spore that drifted into his lab and took root on a culture dish started a chain of events that altered forever the treatment of bacterial infections

The improbable chain of events that led Alexander Fleming to discover penicillin in 1928 is the stuff of which scientific myths are made. Fleming, a young Scottish research scientist with a profitable side practice treating the syphilis infections of prominent London artists, was pursuing his pet theory--that his own nasal mucus had antibacterial effects--when he left a culture plate smeared with Staphylococcus bacteria on his lab bench while he went on a two-week holiday.

When he returned, he noticed a clear halo surrounding the yellow-green growth of a mold that had accidentally contaminated the plate. Unknown to him, a spore of...